A month or so has passed since I upgraded from Tiger to Leopard. Upgrading my iMac to Leopard took less than an hour, and after that everything was ready and fine.
One ugly problem however, after I downloaded and installed the first fat Leopard Security Update (35mb), but that was entirely my own fault. When the reboot took a little longer than I expected, and I assumed nothing was happening at all, I hard yanked the iMac into a reboot, and that was ooh soo wrong. The machine booted up into a fresh new welcome screen, wanted me too enter a new user, but kept rebooting up in a loop.
The result was that I had to do a safe reboot, and a complete reïnstallation from the Leopard CD. Having done that (went smooth again in an hour or so) I fired up Time Machine for the first time on a clean 250 GB external hard disk. After that disk was formatted Leopard started copying files and told me the first back-up process would take more than eight hours. So I went around to do other things and when I came back late at night the iMac was sleeping, and behaved very well when woken up.
Check out the video from Walt Mossberg and read his article: Leopard: Faster, Easier Than Vista.